The easy way to extend it is to get some 3/4" oak dowel from Home Depot or Lowes. Cut 4 piecses the same length (3/4" if you are starting with 3" standoffs) and drill a 1/4" holer down the middle.

If you don't have a table or cutoff saw, cut up some 1/4" ply into 1" squares. Laminate 3 pieces together into a 3/4" thick piece, drill a hole in the center.

Or buy a piece of 1xanything Oak. 1X material is actually 3/4" thick. Cut 4 pieces 1" square, drill a hole you are done.

TF